The present invention relates to a plating bath of the hydrazine type for electroless plating with iridium on the surface of a plated substance. This plating bath can be used, for example, not only for producing a junction of a cation exchange membrane and iridium, which is used for a water electrolytic cell of the macromolecule solid electrolyte type, but also for electroless plating of general-purpose metals such as Cu, Fe, and Ni, valve metals such as Ti, Ta, and Nb, products made of macromolecules, glass, ceramics, etc.
Polyelectrolyte water electrolytic process is known as one of water electrolytic processes used for producing hydrogen and oxygen. This is an electrolytic process using perfluorocarbonsulfonic acid membrane as solid electrolyte and supplying pure water to an anode chamber.
Platinum or a carbon membrane supporting platinum is used for the cathode and a cation exchange membrane joined with platinum and iridium by electroless plating (Japanese Patent Publication No.2-20709), or a cation exchange membrane joined with a membrane supporting mixed oxide of iridium and ruthenium by hot press method (Japanese Patent Laid-open Publication No.52-78788) is used for anode as a cation exchange membrane and an electrode incorporated into this electrolytic cell.
A substance having low overvoltage is used as metal or metal oxide suited for a catalytic electrode. Namely, a platinum electrode is used for hydrogen side and a iridium electrode is used for oxygen side.
A process for producing Pt/M/Pt and Pt/M/Pt-Ir (M is a cation exchange membrane) using electroless plating process is described in Japanese Patent Publication No.2-20709. The plating bath used for this process contains iridium halide and hydrazine, or iridium halide, hydrazine, and hydroxylamine and the pH is 3-10.
When plating is performed by use of this conventional bath liquid, iridium is deposited on platinum layer certainly. However, this process had a defect that the utilization of iridium in the bath liquid is lower than 50% and that the adhesion of deposited iridium is not good.